1. Field of the Invention
This invention generally relates to the drilling and servicing of wells in subterranean formations. In one aspect, it relates to contacting subterranean formations with aqueous well completion and workover fluids which are used in the drilling and servicing of wells.
2. Description of the Prior Art
In the drilling of a well into a subterranean formation, it is necessary to cool the drill bit as it cuts into the formation and to remove drill cuttings away from and out of the borehole. A drilling fluid, therefore, is normally circulated downwardly, through the drill pipe, outwardly through nozzles located in the drill bit and upwardly through the wellbore annulus to the surface. Most drilling fluids, commonly referred to as drilling muds, contain suspended particles of hydrated clay in water. Clay based fluids, in addition to cooling the drill bit and transporting cuttings, prevent cave-ins in poorly consolidated wellbores and gas blowouts in high pressure formations. The colloidal properties of the suspended clay, such as high gel strength and high density, contribute significantly to the functions performed by the clay based drilling fluids. A weighting material such as barite is frequently added to the drilling fluid to increase its density so that formation pressures can be contained.
During the well completion phase of the drilling operation, when the drill bit bores through the producing interval, the insoluble materials of the drilling mud, such as clay and barite, cause a filter cake to be laid down on the formation face. Such a filter cake or sheath tends to seal off the formation and may result in a permanent reduction in the permeability of the producing interval. The embedded clay and barite particles can sometimes be removed by acidization. However, even expensive acid treatments are frequently unable to overcome well damage because clay and barite solids have low solubility in acid.
To avoid impairment of permeability, it is necessary to use a "clean" drilling fluid which will not damage the formation but which has sufficient density to contain formation pressures. Similarly, during well servicing and workover operations, such as gravel packing, in which a permeable, production interval will be contacted with a workover fluid, it is also desirable to employ a dense but nondamaging fluid.
Commonly used nondamaging fluids are high density, aqueous saline solutions such as saturated calcium chloride solutions. Although calcium chloride solutions are nondamaging, they possess a maximum density of only 1.38 g/cc (11.5 pounds per gallon) and may not be suitable for all well treatment operations. Mixtures of calcium bromide and calcium chloride solutions can achieve densities as high as 1.81 g/cc (15.1 pounds per gallon), but such mixtures are very costly. Blends of calcium chloride solutions with other high density solutions such as sodium nitrate, calcium nitrate, and zinc chloride have been proposed, but these solutions are highly corrosive and require the addition of expensive inhibitors. In addition, such solutions may be environmentally hazardous.
The addition of insoluble, suspended solids, such as calcium carbonate, can be used to increase the density of aqueous saline solutions, but these solids can invade the formation and can result in the same type of formation permeability damage which is caused by the clay based fluids. Thus, there is a need for a high density, nondamaging well completion and workover fluid.